I don't understand when I can compute turbulent cfd problems in steady or in transient simulations ? It is impossible to know before the simulation if the flow will be steady or not.

What should I do; try first a steady state case (play with the mesh) and if it does not converge switch to transient case ?

My next step is to compare cfdOF simulations (velocity, pressure, Drag force) with another software; and with literature experimental cases.
To acheive this, I am looking for the simplest 2D case where I can find experiments data. What can I choose, can you tell me a good example for doing this? Maybe you can share your experience on this point also.
I see the Openfoam guide and tutorials which are very helpfull also.

I would choose a cylinder.
-easy to design
-2D-case possible
-different phenoma to investigate (von Karman Vortex, rapid falling drag-coeff at Re=100000 ...)
-try different meshers
-simulate laminar/turbulent, transient/steady conditions
Should be enough homework over christmas
What other cfd-software do you use for comparison of the results?

More seriously I try a simple 2D case of flow past a cylinder. I don't know if my the residual is normal (residual of p higher than 0.1 for all time steps)
The Reynolds number in very low 0.5.
Mesh is max 10 mm, velocity max is 5 mm/s, timestep is 0.5s Courant number should be 0.25.
See my controlDict file attached.

I don't understand if I can constraint automatically the timestep of the solver in using the line "maxCo" in the controlDict file.
Maybe if I don't specify any timestep the solver choose himself the timestep to respect maxCo?

"There are a few different ways of using PIMPLE. With a fixed time step size, it is wise to solve to a particular residual tolerance. One can use a fixed number of iterations if one keeps a limit on the max Courant number. The latter is the mode we use at the moment in CfdOF, as it is not really dependent on appropriate user input of time step size. The first option can give you faster run times, however, and should perhaps be considered."

It means that cfdOF fixe a number of iterations a timestep, no matter if the solution is converged or not and the solver continue the calculation for the next timestep? So the only important value is the maxCo?